World wide analog video standards, such as NTSC and PAL, use interlaced video formats to maximize the vertical refresh rates while minimizing the required transmission bandwidth. In an interlaced video format, a video frame includes a plurality of pixels that are arranged in a plurality of horizontal scan lines. Each frame is split into two video fields. The first of the two fields includes the pixels located in the odd numbered horizontal scan lines, while the second field includes the pixels located in the even numbered scan lines. The interlaced video fields are transmitted sequentially in temporal order to a display system, thereby minimizing the transmission bandwidth.
Typically, a component video signal includes a luma component and chroma components. The luma component represents brightness or luminance information, while the chroma components represent color information, e.g., as color differences, Cb and Cr. The luma component signal and the chroma component signals can be combined to form a composite video signal to minimize further the bandwidth requirements for transmission and to simplify transmission. Both the NTSC and PAL analog video standards support and transmit composite video signals.
In the typical composite video signal, the chroma components are quadrature-modulated. That is, one component of the chroma signal, e.g., Cb, has a different amplitude and is 90 degrees out-of-phase with the other component of the chroma signal, e.g., Cr. The rationale for modulating the chroma subcomponent signals and combining the modulated chroma signal with the baseband luma signal to form a composite video signal is based on the frequency interleaving principle, which provides that both the modulated chroma and baseband luma signal spectra contain individual spectrum lines for stationary video and that those spectrum lines of modulated chroma and baseband luma signals will interleave with each other without overlapping when an appropriate subcarrier frequency is chosen.
During transmission, the quadrature-modulated chroma signal and the baseband luma signal share a portion of the total video signal bandwidth. For example, in NTSC (M) systems, the chroma signal is modulated on a subcarrier frequency of 3.579545 MHz. The chroma signal and luma signal are intermingled within the modulated chroma signal band which extends from roughly 2.3 MHz to 4.2 MHz. In PAL (B/D/G/H/K/N) systems, the chroma signal is modulated on a subcarrier frequency of 4.43361875 MHz. The chroma signal and luma signal are intermingled within the modulated chroma signal band which extends from roughly 3.1 MHz to 5.0 MHz.
When the composite video signal is received by the display system, the signal is decoded by a decoder that separates the modulated chroma signal and the luma signal from the composite signal. One technique for separating the modulated chroma and the luma signals uses a combination of a notch filter passing the luma signal and a bandpass filter passing the modulated chroma signal. Because the filtering of the modulated chroma and the luma signals is performed only in the horizontal domain, this decoding technique is usually called notch filter luma/chroma (Y/C) separation. The modulated chroma signal is subsequently quadrature-demodulated into the two chroma components such as, for example, I and Q signals, or U and V signals. The luma component and the two chroma components can be used in a matrix computation to generate red, green, and blue (RGB) signals when the display system is a television display system.
Another technique for separating the modulated chroma signal and the luma signal uses adaptive line comb filters, i.e., comb filters in the vertical domain using line buffers. This technique is based on the premise that, in a quadrature-modulated composite video signal, the luma signal energy in the vicinity of the modulated chroma band is most probably located near harmonics of the horizontal scanning frequency, while the modulated chroma signal energy is located half horizontal scanning frequency between the luma signal energy peaks for NTSC (M) systems, and one quarter and three quarters horizontal scanning frequency between the luma signal energy peaks for PAL (B/D/G/H/K/N) systems, depending on the relationship between the subcarrier frequency and horizontal scanning frequency. According to this technique, the chroma and luma signals are adaptively averaged across successive scan lines using comb filters based on the presence of vertical transitions in the composite video signal to prevent blurring of the chroma or luma signals in the vertical domain. Because the filtering of chroma and luma signals is done in both the horizontal and vertical domains, this technique is usually called 2-D comb filter Y/C separation, and works particularly well when the luma signal energy is concentrated around harmonics of the horizontal scanning frequency or when the luma signal features are vertical or substantially vertical.
Yet another technique for separating the luma signal from the modulated chroma signal uses motion adaptive frame comb filters, i.e., comb filters in the temporal domain using frame buffers. According to this technique, when motion is detected in the temporal domain, the chroma and the luma signals are adaptively averaged across successive video frames to prevent blurring of chroma or luma signals in the temporal domain. In addition, when vertical transitions within a frame are detected, the chroma and the luma signals are adaptively averaged across scan lines using comb filters to prevent blurring of chroma or luma signals in the vertical domain. Because the filtering of chroma and luma signals is done in the horizontal, vertical, and temporal domains, this technique is usually called 3-D comb filter Y/C separation.
As stated above, the rationale for modulating the chroma components and combining the modulated chroma signal with the luma signal to form a composite video signal is based on the frequency interleaving principle. Nevertheless, the frequency interleaving principle is not applicable for moving video and/or video containing fine diagonal lines. In these cases, the modulated chroma signal spectrum and the luma signal spectrum can and often do overlap with one other. The result is that some degree of mutual interference, i.e., cross-talk, between the luma signal spectrum and chroma signal spectrum can occur.
The term “cross-color” refers to corruption of the modulated chroma signal spectrum caused by cross-talk from the high-frequency luma signal spectrum. When a composite video signal having cross-color is decoded with either a notch filter or a line comb filter, the cross-color can cause visual artifacts that can appear as a coarse rainbow pattern or random colors in image areas having dense diagonal fine lines, such as tiled rooftops, laminated fences, herringbone patterned clothing, and leafy scenery. The term “cross-luma” refers to corruption of the high-frequency luma signal spectrum caused by cross-talk from the modulated chroma signal spectrum. When a composite video signal having cross-luma is decoded with either a notch filter or a line comb filter, the cross-luma can cause visual artifacts that can appear as fine alternating dark and bright dots in image areas having abrupt chroma transitions, such as at the boundaries of contrasting colors, like blue and yellow.
While the visual artifacts caused by cross-color and cross-luma might be tolerable when displayed on a legacy television having a small screen with low brightness and low resolution, such artifacts are highly objectionable when displayed on a modern television having a large screen with high brightness and high resolution. Moreover, the cross-color and cross-luma problems are exacerbated when an advanced image scaling function, typically included in a modern television, enlarges video from a composite video source onto the large-size display with high resolution. In such modern systems, reducing or eliminating cross-color and cross-luma artifacts is highly desirable.
While decoding systems that use notch filter Y/C separation and 2-D comb filter Y/C separation techniques fail to reduce or eliminate cross-color and cross-luma artifacts, decoding systems that use 3-D comb filter Y/C separation techniques can reduce such artifacts in certain situations. For example, because filtering is performed in the horizontal, vertical, and temporal domains, the cross-color and/or cross-luma artifacts can be reduced in stationary portions of an image, such as over fine diagonal lines and along sharp vertical chroma transitions. Nevertheless, implementing a decoder that uses adaptive 3-D frame comb filters can be significantly more expensive then one that uses notch filters or line comb filters because the decoder requires motion detectors and frame buffers. Moreover, because the technique is based on motion detection in the temporal domain, actual motion in the image can be mistakenly interpreted as cross-color and/or cross-luma, and vice versa. In this case, improper filtering can itself produce serious cross-color and cross-luma artifacts.
Other systems for suppressing cross-color and/or cross-luma artifacts implement adaptive cross-color and/or cross-luma suppression techniques that operate only upon modulated chroma signals prior to demodulation. While these systems can be effective when the input signal is a composite video signal, they cannot be used to suppress cross-color and/or cross-luma artifacts in de-modulated baseband component signals. This shortcoming is significant because many modern digital devices are configured to process baseband component signals in the form of one luma (Y) signal and two color differences (Cb and Cr) signals. Such signals are received from, for example, a digital TV (DTV) tuner or a DVD player connected through a serial or parallel digital interface conveying YCbCr component signals. These baseband component signals can exhibit cross-color and/or cross-luma artifacts when the source of the content is taken from a composite video master. In this case, cross-color and/or cross-luma artifacts in the composite video master are irreversibly imprinted and any signals derived from the master necessarily inherit the cross-color and/or cross-luma errors and the associated visual artifacts.
In addition, even when the content source is not taken from a composite video master, the baseband component signals of a video signal can exhibit cross-color and/or cross-luma artifacts when the original component video signal is converted to a composite format during any stage of video processing, such as during production, distribution, transmission, and so forth. In both cases, cross-color and cross-luma detection and suppression must be performed directly on the baseband component signals in order to reduce the objectionable artifacts.
Accordingly, it is desirable to provide a method and system for detecting and suppressing the cross-color and cross-luma present in a baseband component video signal derived from a quadrature-modulated composite video signal. The system should be cost effective and should not require extensive computational and storage resources.